Digital ‘Cordless’ telephones using Time-Division Multiple Access (TDMA) protocols are now commonplace throughout the world with a range of national and international protocol standards providing a basis for a range of low-cost, low-power products to cater for most short-range office and domestic applications.
In a TDMA communications system, time is divided up into a series of notional, fixed-duration frames which are themselves sub-divided into a number of fixed-duration timeslots. Transmitting equipment samples and temporarily stores user information (which might be speech and/or a digital data stream) and then sends it at high burst data rates over a channel defined by a unique pairing of timeslot and a radio frequency carrier. Receiving equipment demodulates the bursts of timeslot data from received radio frequency carrier and assuming errors are not introduced during transmission process, regenerates the original user information at the original sample rate.
A key functional block within any Cordless telephone is the demodulator which typically sits at the boundary between the analogue radio frequency (RF) and digital baseband sections of the receiver.
Demodulation Techniques
There are basically two types of demodulator: those that generate and maintain a local carrier phase reference and demodulate incoming symbols by comparing samples of the received signal phase with that of the local reference; and those that demodulate incoming symbols by making phase shift measurements directly from the received signal itself. The first demodulation method is generally referred to as ‘coherent’ and the second ‘non-coherent’, reflecting the role played by the carrier phase reference.
Both types of demodulator have found widespread use in digital Cordless and digital Cellular telephone systems.
The avoidance of the carrier acquisition and tracking functionality in the non-coherent demodulator increases the design options for low-power, low-complexity receivers and provides some significant operational advantages in fast fading environments and where rapid receiver synchronisation is needed. There are, however, two major drawbacks with this approach. Firstly, non-coherent detection methods are inherently more noisy than their coherent counterparts which reduces receiver sensitivity. Secondly, most channel equalisation methods rely on the existence of a linear channel between transmitter and demodulator and the differential phase detection process of the non-coherent detector introduces a non-linearly which is difficult to overcome without compromising equalisation performance. This basically means that even in only moderately dispersive channels (where multipath causes more than ½ symbol of time dispersion) coherent demodulation can be the only option.
Despite its shortcomings, non-coherent demodulation tends to dominate in short-range Cordless applications where the benefits of low-complexity and fast sync acquisition outweigh the sensitivity and equalisation issues mentioned.
Recent advances in the fields of digital signal processing and very large-scale integration (VLSI) are now allowing the demodulator to be implemented entirely digitally and it is reasonable to assume that forthcoming developments will see the boundary between the analogue and digital sections move further up the analogue receive chain towards the antenna yielding important benefits for equipment cost, reliability and performance.
A common method for realising a digital non-coherent demodulator is by means of a differential detection algorithm implemented on a digital signal processor wherein the modulating symbol sequence is recovered from the received carrier by determining the phase shift over a short period of time (typically a symbol period) and relating the determined phase shift to one of an alphabet of candidate phase shifts each of which denotes a particular transmitted bit or N-bit symbol. One aspect of the present invention is the integration of a such a detector within a receiver architecture.
Radio Architectures
With the move to all-digital demodulation has come the need for wide dynamic range receiver architectures incorporating analogue-to-digital conversion (ADC). Two radio receiver architectures dominate in this area: the types referred to hereafter as ‘Linear’ and ‘Limiting’ summarised in the following description.
In a Linear receiver, the ADC is preceded by one or more stages of variable-gain amplification which are jointly controlled in such a way that the signal level applied to the ADC is high enough to exceed the ADC's quantisation noise floor by a margin appropriate for satisfactory data recovery, but low enough to avoid driving the ADC into saturation. It will be apparent that the wider the dynamic range of the ADC the simpler it is for the gain control circuits to achieve a satisfactory operating condition. Gain control may be applied entirely in hardware for example as an integral part of the analogue RF section or by means of a two-stage mechanism employing a level sensing algorithm in the baseband section coupled to digitally-programmable amplifiers in the radio.
With this type of architecture, there are two popular strategies for gain control; one slow acting, the other more rapid but at the same time more complex. In the former, the signal strength is monitored over a number of TDMA frames and the receiver gain adjusted in steps—typically one per frame—to achieve a desired ADC set point. Despite its relative simplicity, this approach has a drawback in fast fading environments where the signal level may vary widely from one frame to the next. This is a particular problem in TDMA systems which employ slow (or frame rate) frequency hopping. In such circumstances it is necessary to increase the dynamic range of the ADC such that gain uncertainty is accommodated in the same way as normal symbol-to-symbol signal level variations. This can have unacceptable implications for the cost and power consumption of the ADC and, due to the requirement for increased dynamic range in the digital baseband processing, downstream digital demodulation circuits.
The second gain control strategy involves fast-acting automatic gain control. Here, the receiver rain is established independently for each receive TDMA burst from the signal level sensed during the preamble portion of the burst (in this context preamble can be considered to be any non-information-bearing symbol sequence prepended to the portion of the burst carrying user data). The response times of the circuits in the AGC circuit must be tailored to ensure that the desired set point is achieved early in the preamble since otherwise the performance of the other receiver synchronisation functions (which also rely on a stable receive preamble) will be compromised. Of particular concern in this respect are the functions of symbol timing recovery and carrier frequency recovery.
In a Limiting receiver, the variable gain stages in the analogue RF section are replaced by one or more fixed gain stages organised in such a way that the signal presented to the ADC is always clipped (hard limited) irrespective of the level applied at the antenna. This completely eliminates the need for gain control but, due to the loss of sisal envelope information, restricts the choice of demodulator to those which can recover the modulating data sequence from the carrier phase information alone (note that signal phase information is retained at the output of a limiting amplifier). This is not normally a problem except at longer ranges when a degree of equalisation may be necessary to mitigate the fast (symbol-rate) fading effects caused by multipath propagation.
Neither of these two radio receiver architectures is ideal under all practical circumstances, each having its own advantages and disadvantages. The Linear receiver provides the demodulator with a faithful representation of the signal received at the antenna allowing application of all conventional demodulation and equalisation techniques but suffers the disadvantage of needing level sensing and gain control functionality which limits its applicability to slow-fading environments—for which frame-by-frame gain tracking is appropriate—and to TDMA protocols featuring redundant burst headers—where burst-by-burst gain acquisition can be considered.
The Limiting receiver, on the other hand, avoids the gain control issue completely but, due to the loss of signal amplitude information, is inappropriate for use in dispersive, multipath channels requiring equalisation.
Both architectures discussed above are applicable to both coherent and non-coherent demodulators.
Equalisation
The nature of the propagation environment between the antennas of the transmitter and receiver is crucial to the performance of the TDMA radio link and thus the service quality perceived by the user. A particular concern, especially at long ranges, is the effect of what is known in the art as multipath propagation. This is caused when signals reflect from walls, hills, buildings and high-sided vehicles at different ranges resulting in the creation of echoes of the wanted signal at the receiver. The echoes can cause a number of undesirable effects, depending upon their amplitude, phase and delay relative to the wanted component of the signal and each other. These effects range from simple power fades, in channels subject to only small amounts of dispersion, to inter-symbol interference in highly dispersive multipath channels.
In many TDMA radio communication systems, the symbol rate, modulation scheme, transmit power and operating range are deliberately chosen to ensure that, where multipath propagation exists, the delay interval between the arrival of the earliest and latest echo is sufficiently small as a proportion of the transmitted symbol period to cause, at worst, power fades. These can be mitigated with relatively simple techniques such as switched-antenna diversity implemented at the transmitter, receiver or both, such as will be described later.
The dynamic range requirements of both the conventional and diverse receiver configurations are adequately satisfied by the Limiting type of receiver architecture.
However, in order to employ a Linear receiver with the latter it is necessary to use an ADC with dynamic range compatible with worst-case signal level difference between antennas (which could be several tens of Decibels) or have a gain control mechanism capable of establishing the receiver gain independently for each antenna within a time period no greater than that needed to establish the received signal strength. Whilst feasible, both solutions are difficult to achieve in practice and in the flat-fading channel which dominates short-range Cordless applications, the Limiting receiver is evidently the most appropriate of the two candidate receiver architectures.
The choice is not so clear-cut in long-range and non-line-of-sight applications where the channel introduces significant levels of intersymbol interference and where equalisation is warranted. In order to understand the reasons why, it is necessary to consider basic equalisation principles and the impact of the ‘non-linear’ radio channel.
A considerable volume of work has been carried out in the field of multipath equalisation with many technical publications related to the subject. A broad coverage of the subject is presented in the book “Adaptive Filter Theory” by Simon Haykin (Prentice Hall). This book also includes detailed descriptions of the many algorithms employed in current digital radio communications equipment.
Generally speaking, an equaliser models the radio channel as a superposition of variable-delay and variable-phase signal paths and mitigates its effects using one of two strategies. In the first strategy, the equaliser derives and then applies a filter to the received signal which compensates for the time dispersion introduced during transmission. Ideally, the composite channel formed by concatenating the radio channel with the equalisation filter has a unit impulse response allowing the original transmit symbol sequence to be regenerated at its output. Demodulation is then a simple case of comparing received symbols one-by-one with the candidates known to have been transmitted and choosing the most likely according to some predefined selection criterion. Decision feedback equalisers and tapped delay line (also known as linear) equalisers fall into this category.
The second equalisation approach is to derive an estimate of the channel impulse response from the received signal (using known properties of the transmitted sequence) and then to modify the receiver's modulation symbol alphabet to account for its effects. In this case demodulation involves comparing the distorted received symbol samples with the (distorted) transmit prototypes formed by combining the undistorted transmit symbols with the estimated channel impulse response.
Broad comparisons can be drawn between this approach and that used for decoding convolutional binary codes since during transmission, the transmit symbol sequence is convolved with the channel impulse response in the same way that a raw transmit bit stream is encoded by convolving with an encoder polynomial. In both cases, optimum decoder performance requires that decoding is performed by means of maximum-likelihood sequence estimation (MLSE) techniques rather than on the symbol-by-symbol, or bit-by-bit basis used by the decision feedback and tapped delay-line equalisers.
Although the two equalisation strategies are quite different in their detailed treatment of the received signal their underlying principles are identical in the sense that they assume a linear model of the channel. This is a big problem for the Limiting receiver which strips off signal envelope (amplitude) information presenting the equaliser with the worst kind of nonlinear channel. For this reason, equalisation is normally not possible with Limiting receivers.
Nonetheless, given the advantages of the Limiting receiver in the majority of TDMA applications, there is strong motivation for making it compatible with equalisation, both in the case of coherent and non-coherent demodulation.